Effective viscosity from cloud-cloud collisions in three-dimensional global SPH simulations

TL;DR

This study uses 3D SPH simulations to measure the viscous time-scale from cloud-cloud collisions in galactic disks, finding it to be significantly shorter than previous estimates, which impacts models of disk evolution.

Contribution

The paper provides the first direct measurements of cloud-collision viscous time-scales from 3D simulations, challenging prior analytic estimates and clarifying the role of collision efficiency.

Findings

Viscous time-scales are 0.6-16 Gyr, much shorter than earlier estimates.

Collision efficiency significantly affects the viscous time-scale.

Time-scale weakly depends on the number of clouds, indicating resolution effects are limited.

Abstract

Analytic estimates of the viscous time-scale due to cloud-cloud collisions have been as high as thousands of Gyr. Consequently, cloud collisions are widely ignored as a source of viscosity in galactic disks. However, capturing the hydrodynamics of discs in simple analytic models is a challenge, both because of the wide dynamic range and importance of 2D and 3D effects. To test the validity of analytic models we present estimates for the viscous time-scale that are measured from three dimensional SPH simulations of disc formation and evolution. We have deliberately removed uncertainties associated with star-formation and feedback thereby enabling us to place lower bounds on the time-scale for this process. We also contrast collapse simulations with results from simulations of initially stable discs and examine the impact of numerical parameters and assumptions on our work, to constrain possible systematics in our estimates. We find that cloud-collision viscous time-scales are in the range of 0.6-16 Gyr, considerably shorter than previously estimated. This large discrepency can be understood in terms of how the efficiency of collisions is included in the analytical estimates. We find that the viscous time-scale only depends weakly on the number of clouds formed, and so while the viscous time-scale will increase with increasing resolution, this effect is too weak to alter our conclusions.